c# - Sum of child of child in LINQ -

i need in linq query in ef6.

the master table called exam. child examresult. each examresult has question , selected answer. each answer has power (0 if wrong, 1 if correct). if want know total of correct answers, run command:

var examtotal = db.exams.firstordefault(ex => ex.examid == examid).examresults.sum(er => er.answer.power); 

my problem when questions not answered, , nullreferenceexception.

some general nullchecks should trick

var exam = db.exams.firstordefault(ex => ex.examid == examid); var examtotal = exam.examresults.sum(er => er.answer?.power ?? 0); 

...and in case you're not using c# 6, here's version of it:

var exam = db.exams.firstordefault(ex => ex.examid == examid); var examtotal = exam.examresults.sum(er => er.answer != null ? er.answer.power : 0);